There is only 1 artefact that's hard to find. The 3th artefact in chapter 6. All the other artefacts are purple glowing orbs, but this one is not. If you are following the walkthrough you will come to the part where they are talking about a hidden artifact in a room with debris. Pick up the green gas tank with RT. If correct you'll get a notification for finding the artefact.

NOTE: just like the "it takes two" achievement, this one was glitched. We picked up every single artefact while playing online, but no achievement. Following next instruction will gain you the achiev:

- Make sure you still have some ignite ammo- Return to chapter 4 scene "the catacombs"- Go back to the place where you found artifact 1 (Pilot's goggles)- Shoot the red roots to pass and pick up the artifact- Thadaaaa achievement unlocked